In recent years, developments to uses for high-functionality films for packaging and other films for the purpose of improving the barrier property, especially for optical films using optical control such as a polarizing-plate protection film of a liquid crystal display device and an optical compensation film has been in progress.
For example, by alternately laminating a layer having a high refractivity and a layer having a low refractivity a number of times, an optical interference film that selectively reflects light having a specific wavelength or lets the light pass therethrough by the interference of the light between these layers is obtained. By adjusting the wavelength range of the light that is selectively reflected or let pass through the film to a visible light range, such laminated films have been widely used, for example, for a reflective-type polarizing plate or a chromogenic film, a metallic film, or a reflective mirror film. Further, by adjusting the film so as to selectively block near-infrared light, the film can be used as a film that is put up on a window to block the sunlight. Therefore, multi-laminated films are expected to be used for various purposes.
The above-described multi-laminated film is formed by laterally laminating films in the direction of the film thickness by using a multi-layer extrusion technique (Patent literatures 1 and 2). In this multi-layer extrusion technique, various types of thermoplastic materials are supplied from various extrusion machines and forced to pass though a multi-layer manifold die, a multi-layer feed block, and a film die. By doing so, each flow is fused in the feed block, laminated, and fed into the die, and a laminated body is thereby formed.
For example, it is stated in the paragraph [0010] of Patent literature 1 that: a first flow composed of discrete, overlapping layers of the one or more materials is divided into a plurality of branch flows; these branch flows are redirected or repositioned and individually symmetrically expanded and contracted; the resistance to the flow of each of the branch flows is independently adjusted; the branch flows are recombined into a overlapping state; and a second flow in which a extremely large number of discrete, overlapping layers of the one or more materials are distributed in a predetermined gradient or in other distribution states is thereby formed. Meanwhile, Patent literature 2 discloses a method for manufacturing a multi-layer laminated film which is composed of at least two types of thermoplastic resin layers and in which the number of lamination layers is at least two, by using a lamination apparatus having such a structure that the size of each section of a flow path satisfies a predetermined relation in order to reduce the variations among the lamination layers of the multi-layer laminated film.
However, since the manufacturing methods according to Patent literatures 1 and 2 form lamination layers in which the layers are oriented in a lateral direction, there is a problem that thickness variations among layers occur, and thus making it impossible to form uniform layers.
To solve the above-described problem, and as the purposes of multi-layer films are diversified, new multi-layer film manufacturing methods and manufacturing apparatuses different from those in which layers are laminated in the direction of the film thickness have been examined.
For example, Patent literature 3 proposes a method and an apparatus for manufacturing a multi-laminated body oriented in a lengthwise direction, by dividing a viscous high polymer fluid having at least two layers at an early stage, rearranging the divided high polymer fluid, and then recombining the rearranged high polymer fluid. Specifically, Patent literature 3 discloses a manufacturing method of a multi-layer laminated body oriented in a lengthwise direction, including: providing at least a first flow of a first hardenable fluid and a second flow of a second hardenable fluid; combining the first flow with the second flow to supply a combined flow of fluid composed of the first fluid and the second fluid; dividing the combined flow into a plurality of flows, each of the plurality of flows being composed of the first fluid and the second fluid; arranging the plurality of flows laterally adjacent to each other; and fusing the plurality of laterally-adjacent flows to provide a lengthwise-oriented multi-layer laminated body. In this aspect, Patent literature 3 discloses a method including: dividing a first flow into two flows of a first fluid; and combining a second flow, including combining the two flows of the first fluid with the second flow to provide the first flow and a third flow.
However, the manufacturing method according to Patent literatures 3 uses a mechanism for forming an arrangement in which a vertical division and a horizontal arrangement are repeated. Therefore, it is such a mechanism that the flow path from the vertical division to the horizontal arrangement is short in order to reduce the overall length of the apparatus. Therefore, there is a problem that layers at the left/right ends of an obtained lengthwise-oriented article could disappear as the number of times of the divisions and the arrangements increases, and thus causing thickness variations among the layers and disruptions in the lengthwise arrangement. Note that in the apparatus shown in FIG. 7 of Patent literature 3, a value L2/L1, which is defined in this specification, is 0.58.
That is, as shown as Comparative examples 1-2 to 1-5 shown in FIG. 4 of this specification and Comparative examples 2-1 and 2-4 shown in FIGS. 10 and 11 respectively, local flow speed variations occur in a laminated flow flowing through a flow path within the apparatus in which a division(s), a branching(s), a rearrangement(s), and a recombining(s) are carried out. As a result, every time the division, the branching, the rearrangement, and the recombining are repeated, the effect of the variations becomes larger, and thus causing a problem that the cross section of the lamination is disrupted and the layers are vertically lowered. In the above-described defective part, once a defective part occurs in the laminated body, the rate of the defective part increases every time the division and the rearrangement are repeated. Therefore, there is a problem that eventually the rate of the defective part increases so much that the performance of the apparatus is adversely affected.